The purpose of these studies is to determine the mechanisms involved in synaptic transmission in the central nervous system. The initial aim will be to develop the isolated spinal cord of the frog as an experimental model for studying the effects of drugs and other chemical agents that modulate or modify synaptic transmission at central neurons. This preparation allows the functional (e.g., reflex) neuronal activity to be studied by electrophysiological techniques and will also allow morphological and histochemical measurement to be made on the same preparation. Two afferent pathways have been defined: Preliminary experiments, using the histofluorescence and axonal iontophoresis techniques, have shown that one pathway is monaminergic and projects from cells in the brain stem to the somata and dendrites of motoneurons; the second pathway originates from the motor axon collateral, has a cholinergic synapse, and can be activated by antidromic stimulation of the ventral roots. The roles of the cyclic nucleotide system in both transmitter release and transmitter mediation at synapses of spinal neurons in these pathways will be examined. The activity of neurons will be recorded by extracellular and intracellular recording techniques in the presence of agents, such as SQ 20009, which inhibit phosphodiesterase or conversely in the presence of those agents that activate adenylate or guanyl cyclase, such as norepinephrine or acetylcholine. c-AMP and c-GMP levles of the spinal cord will be determined under these conditions and under defined alterations in physiological function.